For some experiments, you may want to know if the signals exceeded a certain level.
This feature is especially useful if the monitor is to be used for a motion alarm
application. The signals that emerge from the signal processing circuits are routed to two
comparators (A3a and A3b). The two comparators determine if the signal has
sufficient amplitude to be considered an alarm condition. Comparator A3a is
referenced above the 2.5v bias point, while the low A3b stage is referenced below the 2.5v
bias voltage. The upper comparator is triggered when the signal swings above the upper
threshold (positive voltage change) and the lower comparator is triggered when the signal
swings below the lower threshold (negative voltage change). The variable resistor R17
symmetrically controls both thresholds and allows a single knob to set the trigger
sensitivity levels. With the values chosen, the comparators can be triggered from voltage
changes as small as +-0.05 volts or as high as +-1.5 volts. An LED connected to the output
of each comparator provides an indication of either a positive or a negative disturbance.
Both LEDs are mounted on the front side of the monitor's metal panel. Diodes D3 and D4
sum the two comparator outputs and with the aid of another comparator A4a drive
the transistor Q1 and Q2. The two transistors Q1 and Q2 are used to
drive the piezoelectric alarm connected to the monitor or an external alarm that is
connect by a shielded cable to the remote alarm output jack at the rear of the monitor.
The external alarm output can sound a remote beeper or close a relay. If desired, the
local alarm feature can also be turned off when the alarm selector switch is placed in the
"off" position. In the off position, the switch disconnects the 9 volt source to
the local beeper alarm and the two indicator LEDs. When the system is operated in the
alarm off mode, the overall current will be much less and will extend the operating time
from the battery.